1. Field of the Invention
This invention relates generally to the separation of a liquid from a liquid/gas mixture, and more particularly to impingement separators. A separator with a curved flow passage provides for removing entrained droplets from a flow mixture.
2. Description of the Prior Art
Droplets are conventionally separated from gas mixtures by diffusion, directed motion in an electric field, and inertial motion. Separators in these forms are already known and may be desirable for the preservation of the environment, reasons of economy, operational safety, as well as plant safety.
Electrostatic precipitation is a process by which the separation of droplets or particles from the gas phase is effected by electrical forces.
A second type of separation process involves diffusional deposition which is based on a combination of inertial, interception and diffusion effects. The inertia effect results when inertia of a droplet causes it to deviate from the streamline of the gas and strike a collection surface. The interception effect results when the streamline passes close by the collection surface which causes individual droplets within the streamline to contact the collection surface. The diffusion effect, also known as Brownian movement, results when droplets move at random within the streamline and are subsequently forced onto the collection surface by the streamline. Electrostatic and diffusional filters are used for the separation of relatively small droplets (in the sub micron range), are associated with high costs, and are only feasible for limited installations.
A third type of separation process is inertial separation which relies on flow diversion and the momentum of the liquid droplet to impinge the droplet on a collection surface. Impingement separators such as cyclone, fin deflector, packed bed, scrubbers, and wire/fiber filters show a large variation with respect to their design and geometry and are, therefore, used for a wide range of applications. Cyclone separators and wet scrubbers are also used in the separation of dust particles. Thus, the described principals of inertial separation applies for both dust particles and droplets. Separation efficiency increases as the relative velocity between the droplet and gas increases. Large droplet diameter, high droplet density and gas velocity, and low gas viscosity provide the most favorable conditions for this separation process. Thus, impingement separators have the following limitations: cost increases as droplet size decreases; liquid is drained from the system by gravity (limiting orientation of the system); high pressure drops result from the high velocity of the gas flowing through the system; liquid flooding occurs when system drainage cannot keep up with the gas flow; and, there is a possibility of re-entrainment of the separated droplets at the liquid/vapor interface due to shear.
Cost and operating considerations of the above-identified separation processes and equipment are summarized within chapter 14, "Practical Experience with Droplet Separators in the Chemical Industry", of Armin Burkholz' book entitled Droplet Separation. This chapter also gives examples of various applications appropriate for the different separation processes and equipment. Chapter 14 of this book is incorporated herein by reference.
Descriptions of several patented separators are listed below. These patented separators use some of the separation processes noted above. These separators have intendant disadvantages in their use in a wide range of applications.
A fin deflector separator is known from U.S. Pat. No. 4,198,215 and comprises a series of sinusoidal shaped fins that extend transversely to the direction of the fluid flow and have liquid collection channels in the longitudinal direction. The collection channels are open at the downstream side of the deflector and are arranged so that they will not produce undesirable eddying of the mixture. The liquid is impinged on the collection surface, requiring high velocity flows for the droplets to overcome the main channel flow. The draining liquid flows in the opposite direction of the main channel flow, under the influence of gravity, in the collection channel. The fins are mounted vertically and, in this case, the maximum permissible stream velocity is lowered. At higher vapor velocities, the liquid may be re-entrained from the baffles and carried out of the separator.
Another type of separator is known from U.S. Pat. No. 4,504,285 for the inertial separation of condensable vapors from a gas mixture. The method and apparatus include a mixture of gas and condensable vapor introduced tangentially into an expansion chamber for isentropic expansion and cooling. Isentropic expansion can be achieved in a nozzle with the expansion energy used to accelerate the gas to very high velocities. A swirling nozzle is used to reach low temperatures through expansion and to utilize the expansion energy by accelerating the tangential component of the flow. The swirling effect means, however, that the nozzle performance will not be isentropic. In addition, cooling will be limited by the size of the inlet relative to the nozzle throat.
The cooled gas mixture enters a cylindrical separation chamber where the centrifugal force impinges the condensed vapors on the wall. The liquid is then removed from the separation chamber through perforations in the wall. Because the vapors are removed in a high pressure region, the dynamic pressure tangential to the wall of the tubular separator will tend to overcome capillary forces and gas will likely enter the liquid chamber. To aid the passage of liquid into the collection chamber it is suggested that a conduit at the upper portion of the chamber be connected to a low pressure region of the expansion chamber to provide a partial vacuum. This is not feasible, however, because the location suggested is a point of stagnation in the rotating gas. A more feasible alternative was suggested in another embodiment that includes a vacuum pump to aid liquid removal.
A certain percentage of the droplets cannot be effectively separated using the centrifugal separation chamber. The droplets experience the highest centrifugal force farthest from the axis of rotation. This value decreases as the distance from the axis of rotation to the droplet decreases. In addition, there is a velocity component directed towards the center of rotation that increases resistance to separation. Thus, as droplet size decreases, the limiting core radius from which no droplets can be separated, increases. For relatively small droplet sizes, such as those resulting from this type of condensation process, extremely high gas velocities and long channel lengths are required to induce separation. These, in turn, lead to high pressure drop and shear induced re-entrainment.
The droplets that do overcome resistance to separation and follow a trajectory, impinge on the collection surface provided the following is true: the separation chamber is sufficient in length for the liquid droplets to coalesce and become large enough to overcome the drag force and the chamber is sufficient in length such that the time required for the droplet trajectory to reach the wall is less than the time required to travel the length of the tubular separator. Once separated, the liquid flows under the influence of gravity into the liquid collection chamber, limiting liquid removal to specific gravitational orientation.
Another type of separator is known from U.S. Pat. No. 4,824,449. The invention generally relates to the transformation of a fluid flow of one type to a fluid flow of another type, and more particularly to cyclone dust separators. The dust-containing gas enters the separator at a relatively high pressure and a turbulent flow is formed that moves spirally within the collection chamber. The dust is collected on the circumferential wall of the chamber due to centrifugal force. The clean gas leaves the separator through an axial passage at a relatively low pressure. The exiting gas flows counter to the dust particles; thus, a higher drag force limits particle separation to large particles and requires high velocity flows. As with fin deflector separators, separation relies on gravity limiting orientation. In addition, large pressure drops and size requirements further limit the use of cyclone separators.
Another type of separator is known from U.S. Pat. No. 3,345,803 for degassing viscose. This invention generally relates to the removal of entrained gasses from liquids, and more particularly to the removal of small gas bubbles from a viscous liquid. A laminar flow of viscose, containing gas bubbles, is established in a closed channel. A shear gradient is imposed upon the viscose between the outer and inner regions of the channel and the gas bubbles migrate inward to the regions of low shear. The gas is collected in the axial direction after the viscose has traveled a sufficient distance along the channel and the gas bubbles have accumulated in the center of the flow channel. The flow channel is formed in a spiral for compactness, but does not aid in the separation of the gas bubbles from the viscose; shear, not secondary flow, aids in the separation process.
Another spirally compact flow channel used for gas/oil separation is described within U.S. Pat. No. 4,345,920. This separator uses a vacuum pump to draw the oil/gas mix through the spiral channel. The separator is used exclusively to deaerate oil where the gas content in solution is 8 to 10 percent by volume. Dissolved air is accomplished by reducing pressure. The deaerator includes a flow passageway with a flow restricting device at its inlet to induce an initial pressure drop. The oil is propelled by a vacuum pump, also lowering the pressure on the oil. This separator is not an inertial separator.